Power amplifier with variable bias impedance

ABSTRACT

Systems and methods including variable power amplifier bias impedance are disclosed. In one aspect, there is provided a power amplifier system including a bias circuit configured to receive a bias voltage and generate a bias signal and a power amplifier stage configured to receive an input radio frequency (RF) signal and generate an output RF signal. The power amplifier system may also include a bias impedance component operatively coupled between the bias circuit and the power amplifier stage. The bias impedance is component configured to receive a control signal and adjust an impedance value of the bias impedance component in response to the control signal.

RELATED APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet, or any correction thereto,are hereby incorporated by reference into this application under 37 CFR1.57.

BACKGROUND Field

Embodiments of the described technology relate to electronic systems,and in particular, to systems including power amplifiers for radiofrequency (RF) electronics.

Description of the Related Technology

RF power amplifiers can be used to boost the power of an RF signalhaving a relatively low power. Thereafter, the boosted RF signal can beused for a variety of purposes, included driving the antenna of atransmitter.

Power amplifiers can be included in mobile phones to amplify an RFsignal for transmission. For example, in mobile phones that communicateusing a cellular standard, a wireless local area network (WLAN)standard, and/or any other suitable communication standard, a poweramplifier can be used to amplify the RF signal. It can be important tomanage the amplification of an RF signal, as amplifying the RF signal toan incorrect power level or introducing significant distortion of theoriginal RF signal can cause a wireless device to transmit out of bandor violate compliance with accepted standards. Biasing of a poweramplifier device is an important part of managing the amplificationbecause it can determine the voltage and/or current operating point ofthe amplifying devices within the power amplifier.

There is a need for improved power amplifier systems. Furthermore, thereis a need for improving power amplifier biasing.

SUMMARY

Aspects of this disclosure relate to techniques and electronic systemswhich can be used to improve power amplifier biasing. For example, inone aspect, a power amplifier system, includes a bias circuit configuredto receive a bias voltage and generate a bias signal; a power amplifierstage configured to receive an input radio frequency signal and generatean output radio frequency signal. The system further includes a biasimpedance component operatively coupled between the bias circuit and thepower amplifier stage, the bias impedance component configured toreceive a control signal and adjust an impedance value of the biasimpedance component in response to the control signal.

A method of adjusting a gain of a power amplifier stage is providedaccording to another aspect of the disclosure. The method includesreceiving, at a bias circuit, a bias voltage, generating, by the biascircuit, a bias signal based on the bias voltage, and receiving, at abias impedance component, the bias voltage and a control signal. Themethod further includes adjusting, at the bias impedance component, animpedance value of the bias impedance component based on the receivedcontrol signal, receiving, at a power amplifier stage, an input radiofrequency signal and the bias voltage from the bias impedance component,and generating an output radio frequency signal based on the input radiofrequency signal and the bias voltage.

A mobile device is provided according to yet another aspect of thedisclosure. The mobile device includes a power amplifier configured toamplify an input radio frequency signal and generate an output radiofrequency signal and a modulator configured to generate a radiofrequency transmit signal based on the output radio frequency signal.The power amplifier includes a bias circuit configured to receive a biasvoltage and generate a bias signal, a power amplifier stage configuredto receive the input radio frequency signal and generate the outputradio frequency signal, and a bias impedance component operativelycoupled between the bias circuit and the power amplifier stage. The biasimpedance component is configured to receive a control signal and adjustan impedance value of the bias impedance component in response to thecontrol signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a power amplifier module for amplifyinga radio frequency (RF) signal.

FIG. 2 is a schematic block diagram of an example wireless device thatcan include one or more of the power amplifier modules of FIG. 1.

FIG. 3 is a schematic block diagram of one example of a power amplifiersystem.

FIG. 4 is a schematic block diagram of another example of a poweramplifier system in accordance with aspects of this disclosure.

FIG. 5A is a schematic block diagram of yet another example of a poweramplifier system in accordance with aspects of this disclosure.

FIG. 5B is a schematic block diagram of still yet another example of apower amplifier system in accordance with aspects of this disclosure.

FIGS. 6A-6F are graphs illustrating the effect of changes in the biasimpedance on power amplifier characteristics in accordance with aspectsof this disclosure.

FIG. 7 is a graph illustrating the gain at the output stage of a poweramplifier as a function of the output power in accordance with aspectsof this disclosure.

FIGS. 8A-8D illustrate a number of power amplifier characteristics inaccordance with aspects of this disclosure.

FIG. 9 illustrates an embodiment of a multi-stage power amplifier systemin accordance with aspects of this disclosure.

FIG. 10 is a schematic block diagram of another example of a poweramplifier system in accordance with aspects of this disclosure.

FIG. 11 is a schematic block diagram of yet another example of a poweramplifier system in accordance with aspects of this disclosure.

DETAILED DESCRIPTION

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

Apparatus and methods for biasing power amplifiers are disclosed herein.In certain implementations, a power amplifier system including a poweramplifier and a bias circuit is provided. The power amplifier can beused to amplify a radio frequency (RF) signal for transmission, and thebias circuit can be used to generate a bias voltage for biasing thepower amplifier. The power amplifier bias circuit can receive an enablesignal that can be used to enable or disable the power amplifier so asto pulse the power amplifier's output.

As is described in greater detail below, the bias impedance of thesignal provided to the power amplifier by the bias circuit may affectcertain characteristics of the power amplifier, and in particular, thepower amplifier gain characteristics. Thus, the design and selection ofthe bias impedance provided by the bias circuit is an important designcharacteristic to be considered during power amplifier system design.Aspects of this disclosure relate to a power amplifier system which mayhave an adjustable bias impedance, which can be used to select poweramplifier gain characteristics depending on the design and/orapplication requirements of the power amplifier system.

Overview of Examples of Power Amplifier Systems

FIG. 1 is a schematic diagram of a power amplifier module 10 foramplifying a radio frequency (RF) signal. The illustrated poweramplifier module (PAM) 10 can be configured to amplify an RF signalRF_IN to generate an amplified RF signal RF_OUT. As described herein,the power amplifier module 10 can include one or more power amplifiers,including, for example, multi-stage power amplifiers.

FIG. 2 is a schematic block diagram of an example wireless or mobiledevice 11 that can include one or more of the power amplifier modules ofFIG. 1. The wireless device 11 can include power amplifier bias circuitsimplementing one or more features of the present disclosure.

The example wireless device 11 depicted in FIG. 2 can represent amulti-band and/or multi-mode device such as a multi-band/multi-modemobile phone. In the illustrated configuration, the wireless device 11includes switches 12, a transceiver 13, an antenna 14, power amplifiers17, a control component 18, a computer readable medium 19, a processor20, and a battery 21.

The transceiver 13 can generate RF signals for transmission via theantenna 14. Furthermore, the transceiver 13 can receive incoming RFsignals from the antenna 14.

It will be understood that various functionalities associated with thetransmission and receiving of RF signals can be achieved by one or morecomponents that are collectively represented in FIG. 2 as thetransceiver 13. For example, a single component can be configured toprovide both transmitting and receiving functionalities. In anotherexample, transmitting and receiving functionalities can be provided byseparate components.

Similarly, it will be understood that various antenna functionalitiesassociated with the transmission and receiving of RF signals can beachieved by one or more components that are collectively represented inFIG. 2 as the antenna 14. For example, a single antenna can beconfigured to provide both transmitting and receiving functionalities.In another example, transmitting and receiving functionalities can beprovided by separate antennas. In yet another example, different bandsassociated with the wireless device 11 can be provided with differentantennas.

In FIG. 2, one or more output signals from the transceiver 13 aredepicted as being provided to the antenna 14 via one or moretransmission paths 15. In the example shown, different transmissionpaths 15 can represent output paths associated with different bandsand/or different power outputs. For instance, the two example poweramplifiers 17 shown can represent amplifications associated withdifferent power output configurations (e.g., low power output and highpower output), and/or amplifications associated with different bands.Although FIG. 2 illustrates a configuration using two transmission paths15, the wireless device 11 can be adapted to include more or fewertransmission paths 15.

The power amplifiers 17 can be used to amplify a wide variety of RFsignals. For example, one or more of the power amplifiers 17 can receivean enable signal that can be used to pulse the output of the poweramplifier to aid in transmitting a wireless local area network (WLAN)signal or any other suitable pulsed signal. In certain configurations,one or more of the power amplifiers 17 are configured to amplify a Wi-Fisignal. Each of the power amplifiers 17 need not amplify the same typeof signal. For example, one power amplifier can amplify a WLAN signal,while another power amplifier can amplify, for example, a Global Systemfor Mobile (GSM) signal, a code division multiple access (CDMA) signal,a W-CDMA signal, a Long Term Evolution (LTE) signal, or an EDGE signal.

One or more features of the present disclosure can be implemented in theforegoing example modes and/or bands, and in other communicationstandards.

In FIG. 2, one or more detected signals from the antenna 14 are depictedas being provided to the transceiver 13 via one or more receiving paths16. In the example shown, different receiving paths 16 can representpaths associated with different bands. Although FIG. 2 illustrates aconfiguration using four receiving paths 16, the wireless device 11 canbe adapted to include more or fewer receiving paths 16.

To facilitate switching between receive and transmit paths, the switches12 can be configured to electrically connect the antenna 14 to aselected transmit or receive path. Thus, the switches 12 can provide anumber of switching functionalities associated with an operation of thewireless device 11. In certain configurations, the switches 12 caninclude a number of switches that provide functionalities associatedwith, for example, switching between different bands, switching betweendifferent power modes, switching between transmission and receivingmodes, or some combination thereof. The switches 12 can also provideadditional functionality, including filtering and/or duplexing ofsignals.

FIG. 2 shows that in certain configurations, a control component 18 canbe provided for controlling various control functionalities associatedwith operations of the switches 12, the power amplifiers 17, and/orother operating component(s), such as bias circuits. Non-limitingexamples of the control component 18 are described herein in greaterdetail.

In certain configurations, a processor 20 can be configured tofacilitate implementation of various processes described herein. Theprocessor 20 can operate using computer program instructions. Thesecomputer program instructions may be provided to the processor 20.

In certain configurations, these computer program instructions may alsobe stored in a computer-readable memory 19 that can direct the processor20 or other programmable data processing apparatus to operate in aparticular manner.

The battery 21 can be any suitable battery for use in the wirelessdevice 11, including, for example, a lithium-ion battery.

FIG. 3 is a schematic block diagram of one example of a power amplifiersystem 26. The illustrated power amplifier system 26 includes theswitches 12, the antenna 14, the battery 21, a directional coupler 24, apower amplifier bias circuit 30, a power amplifier 32, and a transceiver33. The illustrated transceiver 33 includes a baseband processor 34, anI/Q modulator 37, a mixer 38, and an analog-to-digital converter (ADC)39. Although not illustrated in FIG. 3 for clarity, the transceiver 33can include circuitry associated with receiving signals over one or morereceive paths.

The baseband signal processor 34 can be used to generate an I signal anda Q signal, which can be used to represent a sinusoidal wave or signalof a desired amplitude, frequency, and phase. For example, the I signalcan be used to represent an in-phase component of the sinusoidal waveand the Q signal can be used to represent a quadrature component of thesinusoidal wave, which can be an equivalent representation of thesinusoidal wave. In certain implementations, the I and Q signals can beprovided to the I/Q modulator 37 in a digital format. The basebandprocessor 34 can be any suitable processor configured to process abaseband signal. For instance, the baseband processor 34 can include adigital signal processor, a microprocessor, a programmable core, or anycombination thereof. Moreover, in some implementations, two or morebaseband processors 34 can be included in the power amplifier system 26.

The I/Q modulator 37 can be configured to receive the I and signals fromthe baseband processor 34 and to process the I and Q signals to generatean RF signal. For example, the I/Q modulator 37 can include DACsconfigured to convert the I and Q signals into an analog format, mixersfor upconverting the I and Q signals to radio frequency, and a signalcombiner for combining the upconverted I and Q signals into an RF signalsuitable for amplification by the power amplifier 32. In certainimplementations, the I/Q modulator 37 can include one or more filtersconfigured to filter frequency content of signals processed therein.

The power amplifier bias circuit 30 can receive an enable signal ENABLEfrom the baseband processor 34 and a battery or power high voltageV_(CC) from the battery 21, and can use the enable signal ENABLE togenerate a bias voltage V_(BIAS) for the power amplifier 32.

Although FIG. 3 illustrates the battery 21 directly generating the powerhigh voltage V_(CC), in certain implementations the power high voltageV_(CC) can be a regulated voltage generated by a regulator that ispowered using the battery 21. In one example, a switching regulator,such as a buck and/or boost converter, can be used to generate the powerhigh voltage V_(CC).

The power amplifier 32 can receive the RF signal from the I/Q modulator37 of the transceiver 33, and can provide an amplified RF signal to theantenna 14 through the switches 12.

The directional coupler 24 can be positioned between the output of thepower amplifier 32 and the input of the switches 12, thereby allowing anoutput power measurement of the power amplifier 32 that does not includeinsertion loss of the switches 12. The sensed output signal from thedirectional coupler 24 can be provided to the mixer 38, which canmultiply the sensed output signal by a reference signal of a controlledfrequency so as to downshift the frequency content of the sensed outputsignal to generate a downshifted signal. The downshifted signal can beprovided to the ADC 39, which can convert the downshifted signal to adigital format suitable for processing by the baseband processor 34.

By including a feedback path between the output of the power amplifier32 and the baseband processor 34, the baseband processor 34 can beconfigured to dynamically adjust the I and Q signals to optimize theoperation of the power amplifier system 26. For example, configuring thepower amplifier system 26 in this manner can aid in controlling thepower added efficiency (PAE) and/or linearity of the power amplifier 32.

Power Amplifier Bias

A theoretically ideal power amplifier has linear gain and phasecharacteristics regardless of the input or output power of the poweramplifier. The gain characteristics of a power amplifier may be plottedon an AM/AM graph which illustrates the change in output amplitude vs.the change in input amplitude. As used herein, AM may refer to amplitudevariation. The theoretically ideal power amplifier has a variation inthe AM/AM plot of 0 dB/dB. The phase characteristics of a poweramplifier may be plotted on an AM/PM graph which illustrates the changein output phase vs. the change in input amplitude. As used herein, PMmay refer to phase variations. Similar to the ideal AM/AMcharacteristics, the theoretically ideal power amplifier has a variationin the AM/PM plot of 0 dB/dB.

Since real-world power amplifiers cannot achieve the flat gain and phasecharacteristics of the theoretical ideal power amplifier, one importantaspect of power amplifier design is improving the linearity of the gainand phase characteristics of the power amplifier. In certainimplementations, there may be trade-offs in terms of the achievablelinearity of the gain and phase characteristics of a power amplifierwithout negatively affecting the output power and efficiency of thepower amplifier. In a multi-stage power amplifier system, the gain andphase characteristics of the power amplifiers at each stage in thesystem may be selected such that the overall gain and phasecharacteristics are substantially liner.

FIG. 4 is a schematic block diagram of another example of a poweramplifier system in accordance with aspects of this disclosure. Inparticular, the illustrated power amplifier system 27 includes a poweramplifier bias circuit 30, a power amplifier stage 41, a current source75, and a bias impedance component 80. The power amplifier bias circuit30 may include a transistor 71 and two diodes 73 and 74. The componentsof the power amplifier bias circuit 30, together with the current source75, may be arranged so as to generate a current mirror which mirrors thecurrent generated by the power amplifier stage 41. The output of thepower amplifier bias circuit 30 is supplied to the bias impedancecomponent 80, which is turn coupled to the power amplifier stage 41 toprovide a bias signal thereto.

The power amplifier stage 41 is configured to receive both an input RFsignal RFIN and the bias signal from the bias impedance component 80.Based on the received signals, the power amplifier stage 41 isconfigured to generate an output RF signal RFOUT. The power amplifierstage 41 is configured to generate the output RF signal RFOUT as anamplified version of the input RF signal RFIN having gain and phasecharacteristics which approach that of the theoretical ideal poweramplifier (e.g., the gain and phase characteristics are designed to bewithin a threshold range of 0 dB/dB). The power amplifier stage 41includes a transistor 61, a plurality of capacitors 52, 65, and 64, anda plurality of inductors 53, 63, and 66. The capacitors 52, 65, and 64and inductors 53, 63, and 66 couple the transistor 61 to receive theinput RF signal RFIN, and a power supply voltage Vcc and to generate theoutput RF signal RFOUT.

The base of the transistor 61 receives the bias signal generated by thepower amplifier bias circuit 30 via the bias impedance component 80. Incertain implementations, the impedance value of the bias impedancecomponent 80 may be selected to dominate the overall bias impedanceapplied to the base of the transistor 61. In certain implementations thebias impedance applied to the base of the transistor 61 may be equal tothe sum of the output impedance of the transistor 71 and the impedancevalue of the bias impedance component 80. Thus, the impedance value ofthe bias impedance component 80 may be selected to dominate theimpedance value of the transistor 71 (e.g., the impedance value of thebias impedance component 80 may be one or more orders of magnitudelarger than the impedance value of the transistor 71). In exemplaryembodiments, the output impedance of the transistor 71 is inverselyrelated to the trans conductance of the transistor 71, which may resultin an output impedance for the transistor 71 on the order of 10Ω.

In certain embodiments, the transistor 61 may comprise a heterojunctionbipolar transistor (HBT), which may adapted for the high frequencysignals the power amplifier system 27 is designed to receive andamplify. In particular, HBTs may have high performance and efficiencyfor RF power amplification as used in the embodiments disclosed herein.In order to properly generate the mirror current in the power amplifierbias circuit, the transistor 71 may also comprise an HBT in certainembodiments.

One technique for adjusting the gain and phase characteristics of apower amplifier may be to select a fixed bias impedance to be applied tothe bias signal supplied to the base of the transistor 61. Theparticular bias impedance may be selected during design and developmentof the power amplifier stage 41 and implemented by selecting theimpedance value of the bias impedance component 80. For example, theimpedance value of the bias impedance component 80 may be selected viadie variants and/or laser trim-able resistors. However, the impedancevalue of the bias impedance component 80 may be selected to adjustand/or improve the linearity of the gain and phase characteristics forthe power amplifier system 27 for a single power level, modulation, andfrequency. Thus, if the power amplifier system 27 is used at a differentpower level, modulation, and/or frequency from the values used inselecting the value of the bias impedance component 80, the linearity ofthe gain and/or phase characteristics of the power amplifier system maysuffer.

Accordingly, certain aspects of this disclosure relate to the use of avariable bias impedance component which may be applied to bias a poweramplifier. FIG. 5A is a schematic block diagram of yet another exampleof a power amplifier system in accordance with aspects of thisdisclosure. Components of the power amplifier system 28 illustrated inFIG. 5A which are similar to, or substantially the same as, componentsin the power amplifier system 27 illustrated in FIG. 4 are representedby the same reference numerals and detailed descriptions thereof may beomitted for the sake of clarity.

As shown in FIG. 5A, the power amplifier system 28 includes a poweramplifier bias circuit 30, a power amplifier stage 41, a current source75, and a bias impedance component 81. In FIG. 5A, the bias impedancecomponent may be implemented as a variable bias impedance component 85.Similar to the power amplifier system 27 of FIG. 4, in the embodiment ofFIG. 5A, the output of the power amplifier bias circuit 30 is suppliedto the variable bias impedance component 85, which is in turn coupled tothe power amplifier stage 41 to provide a bias signal thereto. Thevariable bias impedance component 85 may be configured to receive acontrol signal CTRL configured to adjust the impedance value of thevariable bias impedance component 85. Accordingly, the impedance valueof the variable bias impedance component 85 may be adjusted based on thevoltage of the control signal CTRL. In the illustrated example, thevariable bias impedance component 85 can be implemented using anyvariable impedance element (e.g., a variable resistor) controllable by acontrol signal CTRL.

Certain variable impedance technologies may not be practical for allpower amplifier systems 28, and in particular, for power amplifierswhich may be implemented in mobile phones to amplify RF signals fortransmission. As discussed above, in certain RF power amplifierapplications, it is desirable to implement the transistor 61 of theamplifier stage 41 as an HBT transistor since HBTs may have theperformance and efficiency characteristics that are desirable for use inan RF power amplifier. In come semiconductor fabrication techniques, itmay be difficult to combine different device technologies on a singlesemiconductor die. For example, the combination of an HBT with a fieldeffect transistor (FET) onto the same semiconductor die may not resultin a device having the desired transistor properties. As discussed inU.S. Pat. No. 9,105,488 B2, patented on Aug. 11, 2015, herebyincorporated by reference in its entirety, some attempts to integrate aFET into a GaAs HBT process have resulted only in an n-type FET device.However, recent developments in the fabrication technology asexemplified by U.S. Pat. No. 9,105,488 B2 have enabled the fabricationof HBTs and FETs onto a single semiconductor die.

Using techniques which enable the fabrication of a semiconductor devicehaving both HBT and FET technologies, one embodiment of the poweramplifier 28 of FIG. 5A is illustrated in FIG. 5B. FIG. 5B is aschematic block diagram of still yet another example of a poweramplifier system in accordance with aspects of this disclosure. Thepower amplifier 29 illustrated in FIG. 5B includes a power amplifierbias circuit 30, a power amplifier stage 41, and a current source 75,each of which may be the same as or similar to those discussed above inconnection with FIG. 5A. The power amplifier 29 further includes biasimpedance component 81 which includes an FET 90 in place of the variablebias impedance component 85 of FIG. 5A. The FET 90 may be configured toreceive a control signal CTRL at a gate thereof via an optional resistor95. The control signal is configured to adjust the impedance value ofthe FET 90. Accordingly, the impedance value of the FET 90 may beadjusted by selecting the voltage of the control signal CTRL. Inexemplary embodiments, the FET 90 may be operated in triode via theselection of the voltage of the control signal CTRL. As understood bythose skilled in the art, the triode region of the FET 90 may refer to arange of voltages which can be applied to the gate of the FET 90 suchthat the FET 90 operates in a fashion similar to a resistor (e.g., theFET 90 may have a substantially linear response while operated intriode). Thus, the impedance of the FET 90 may be controlled by thecontrol signal CTRL when the FET 90 is operated within the trioderegion.

The value of the bias impedance supplied to the base of the transistor61 of the amplifier stage 41 may affect the gain and phasecharacteristics of the power amplifier 28 or 29. In particular, FIGS.6A-6F are graphs illustrating the effect of changes in the biasimpedance on power amplifier characteristics in accordance with aspectsof this disclosure. FIGS. 6A-60 illustrate power amplifiercharacteristics when a relatively high impedance is applied to the baseof the transistor of a power amplifier while FIGS. 6D-6F illustratepower amplifier characteristics when a relatively low impedance isapplied to the base of the transistor of a power amplifier. FIGS. 6A-6Fare meant only to illustrate how increasing or decreasing the biasimpedance value affects the power amplifier characteristics, and thus,the specific values of the bias impedance value which result in theillustrated graphs are not limiting. In certain embodiments, a “low”impedance value may be substantially zero impedance while a “high”impedance value may be an infinite impedance value.

FIGS. 6A and 6D are graphs which show the base-collector voltage (V) andbase current (A) of a transistor (such as transistor 61 of FIG. 5A or5B) in a power amplifier as a function of the input power (dBm) to thetransistor under “high” and “low” bias impedance. Of note in the highbias impedance of FIG. 6A, the base-collector voltage (V) decreases withincreasing input power while in the low bias impedance of FIG. 6D, thebase-collector voltage (V) is substantially fixed with increasing inputpower.

FIGS. 6B and 6E are graphs which show the gain (dB) and output power(dBm) of the transistor in a power amplifier as a function of the inputpower (dBm) to the transistor under “high” and “low” bias impedance.Here, in the high bias impedance of FIG. 6B, the gain “compresses” ordecreases with increasing input power while in the low bias impedance ofFIG. 6E, the gain increases with increasing input power. Thus, byselecting the bias impedance with a value between the illustrated “high”and “low” values, the gain characteristics of the power amplifier can beimproved by flattening the gain out, thereby improving gain linearity.

FIGS. 6C and 6F are graphs which show the output current (A) of thetransistor in a power amplifier as a function of the input power (dBm)to the transistor under “high” and “low” bias impedance. Here, in thehigh bias impedance of FIG. 6C, the DC output current is substantiallyfixed with respect to input power while in the low bias impedance ofFIG. 6F, the DC output current increases with increasing input power.

FIG. 7 is a graph illustrating the gain at the output stage of a poweramplifier as a function of the output power in accordance with aspectsof this disclosure. Various gain curves are illustrated in FIG. 7 atdifferent impedance bias levels which range from an impedance of 5Ω toan impedance of 1000Ω as shown in the legend. As the bias impedance isincreased the gain drops with increasing output power. In theillustrated embodiment, an impedance bias of 50Ω may be selected toprovide a substantially flat gain, However, other power amplifiertopologies may result in different output stage gain plots, and thus,the specific impedance bias which results in a substantially flat gainmay depend on the particular implementation of the power amplifier.

FIGS. 8A-8D illustrate a number of power amplifier characteristics inaccordance with aspects of this disclosure. In particular, FIG. 8Aillustrates the gain (dB) as a function of output power for a poweramplifier at a number of different bias impedance values; FIG. 8Billustrates the phase (deg) as a function of output power for a poweramplifier at the different bias impedance values; FIG. 8C illustratesthe power amplifier efficiency (%) as a function of output power for apower amplifier at the different bias impedance values; and FIG. 8Dillustrates the transistor collector current (mA) as a function ofoutput power for a power amplifier at the different bias impedancevalues.

As shown in FIGS. 8B-8C, the bias impedance value does not have asignificant effect on the phase, power amplifier efficiency, ortransistor collector current characteristics of the power amplifier.However, as shown in FIG. 8A, the bias impedance value affects the gaincharacteristics of the power amplifier, with increasing gate controlvoltage (e.g., increasing bias impedance) resulting in an increase ingain as a function of output power. Thus, the adjustment of the biasimpedance may be an effective tool for adjusting the gaincharacteristics of the power amplifier without significantly affectingthe phase, power amplifier efficiency, and transistor collector currentcharacteristics of the power amplifier.

FIG. 9 illustrates an embodiment of a multi-stage power amplifier systemin accordance with aspects of this disclosure. In particular, the poweramplifier system 121 of FIG. 9A includes a first power amplifier 120 anda second power amplifier 125 connected in series between an RF inputport RFIN and an RF output port RFOUT. Each of the first and secondpower amplifiers 120 and 125 may be the same as or similar to the poweramplifier 29 illustrated in FIG. 5B. Thus, a detailed described of eachof the constituent components will not be provided. In the embodiment ofFIG. 9, the bias impedance value supplied to the base of the transistors61 of each of the power amplifiers 120 and 125 may be individuallyselected in accordance with the particular implementations of theamplifier stages 41 and the power amplifier bias circuits 30 in each ofthe first and second power amplifiers 120 and 125. Further, the biasimpedances selected by the gate control voltages Gate CTRL applied tothe respective amplifier stages 41 may be selected such that the overallgain of the power amplifier system 121 (e.g., the gain at the outputport RFOUT for a signal applied to the input port RFIN) is sufficientlyflat. Thus, in certain implementations, the gains of each of the firstand second power amplifiers 120 and 125 may not be substantially flat solong as the overall gain of the power amplifier system 121 has avariation that is less than a threshold value from the ideal variationof 0 dB/dB.

FIG. 10 is a schematic block diagram of another example of a poweramplifier system in accordance with aspects of this disclosure. Thepower amplifier 130 illustrated in FIG. 10 includes a power amplifierbias circuit 30, a power amplifier stage 41, and a current source 75,each of which may be the same as or similar to those discussed above inconnection with FIG. 5A or FIG. 5B. The power amplifier 130 furtherincludes a bias impedance component 81 which includes a pair of FETs 91and 92 in place of the single FET 90 of FIG. 5B. The control signal CTRLmay be applied to the gate of each of the FETs 91 and 92 via respectiveresistors 95 and 97. Depending on the implementation, the use of twoFETs 91 and 92 may increase the range of values for the impedance biasprovided by the combination of the FETs 91 and 92 while maintaining theFETs 91 and 92 in triode. Although not illustrated in FIG. 10, asimilarly structured circuit (including two FETs and input resistors)may be included in the power amplifier bias circuit to preserve thecorrect current mirror ratio.

FIG. 11 is a schematic block diagram of yet another example of a poweramplifier system in accordance with aspects of this disclosure. Thepower amplifier 131 illustrated in FIG. 11 is similar to the poweramplifier 130 illustrated in FIG. 10, except for the inclusion of threeor more FETs 91 to 92 in the bias impedance component 81, where theinclusion of additional FETs is illustrated by the ellipses, Byincluding three or more FETs 91 to 92 in the bias impedance component81, the range of bias impedance values that can be generated whilemaintaining the FETs 91 and 92 in triode increases. As described abovein connection with FIG. 10, the same structure (including the samenumber of FETs and resistors) can be included in the power amplifierbias circuit to preserve the current mirror ratio.

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Likewise, the word “connected”, as generally used herein, refers to twoor more elements that may be either directly connected, or connected byway of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the above Detailed Description using the singular or pluralnumber may also include the plural or singular number respectively. Theword “or” in reference to a list of two or more items, that word coversall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “e.g.,” “for example,” “such as” and the like,unless specifically stated otherwise, or otherwise understood within thecontext as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above, The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A power amplifier system comprising: a biascircuit configured to receive a bias voltage and generate a bias signal;at least one power amplifier stage configured to receive an input radiofrequency signal and generate an output radio frequency signal; and atleast first and second bias impedance components operatively coupledbetween at least one bias circuit and at least one power amplifierstage, the first and second bias impedance components bias the at leastone power amplifier stage with a variable impedance based on a controlsignal that is received separately from the bias circuit.
 2. The poweramplifier system of claim 1 wherein first and second bias impedancecomponents include at least first and second transistors and the controlsignal has a voltage configured to operate the first and secondtransistors in a triode region.
 3. The power amplifier system of claim 2wherein the first and second transistors include first and second fieldeffect transistors and the at least one power amplifier stage includesat least a heterojunction bipolar transistor configured to amplify theinput radio frequency signal.
 4. The power amplifier system of claim 3wherein the first and second field effect transistors and theheterojunction bipolar transistor are fabricated on a singlesemiconductor die.
 5. The power amplifier system of claim 1 wherein animpedance value of the first and second bias impedance components isselected such that variation in a gain of the at least one poweramplifier stage is less than a threshold value from 0 dB/dm.
 6. Thepower amplifier system of claim 1 wherein the first and second biasimpedance components are connected in serial.
 7. The power amplifiersystem of claim 1 further comprising three or more bias impedancecomponents.
 8. The power amplifier system of claim 1 wherein the firstbias impedance component is connected between a first bias circuit and afirst power amplifier stage and the second bias impedance component isconnected between a second bias circuit and a second power amplifierstage.
 9. A method of adjusting a gain of a power amplifier stage,comprising: generating with at least one bias circuit, at least one biassignal; receiving, at least first and second bias impedance components,the bias signal and a control signal, the control signal being receivedseparately from the bias circuit; adjusting a variable impedance of thefirst and second bias impedance components based on the control signal;and biasing at least one power amplifier stage with the variableimpedance to vary the generation of an output radio frequency signal.10. The method of claim 9 wherein the first and second bias impedancecomponents include first and second transistors, and the control signaloperates the first and second transistors in a triode region.
 11. Themethod of claim 10 wherein the first and second transistors includefield effect transistors and the at least one power amplifier stageincludes a heterojunction bipolar transistor, the method furthercomprising amplifying, at the heterojunction bipolar transistor, theoutput radio frequency signal.
 12. The method of claim 11 wherein thefield effect transistors and the heterojunction bipolar transistor arefabricated on a single semiconductor die.
 13. The method of claim 9further comprising selecting a voltage of the control signal to producethe impedance value of the first and second impedance components suchthat variation in a gain of the at least one power amplifier stage isless than a threshold value from 0 dB/dm.
 14. The method of claim 9further comprising connecting the first and second bias impedancecomponents in serial.
 15. The method of claim 9 further comprisingconnecting three or more bias impedance components together.
 16. Themethod of claim 9 further comprising connecting the first bias impedancecomponent between a first bias circuit and a first power amplifier stageand connecting the second bias impedance component between a second biascircuit and a second power amplifier stage.
 17. A mobile devicecomprising: a bias circuit configured to receive a bias voltage andgenerate a bias signal; at least one power amplifier stage configured toreceive an input radio frequency signal and generate an output radiofrequency signal; at least first and second bias impedance componentsoperatively coupled between at least one bias circuit and at least onepower amplifier stage, the first and second bias impedance componentsbias the at least one power amplifier stage with a variable impedancebased on a control signal that is received separately from the biascircuit; and a modulator configured to generate a radio frequencytransmit signal based on the output radio frequency signal.
 18. Themobile device of claim 17 wherein the first and second bias impedancecomponents are connected in serial.
 19. The mobile device of claim 17further comprising three or more bias impedance components.
 20. Themobile device of claim 17 wherein the first bias impedance component isconnected between a first bias circuit and a first power amplifier stageand the second bias impedance component is connected between a secondbias circuit and a second power amplifier stage.